The present invention concerns a method for preparing metal borates and more particularly thin films of metal borate, e.g. barium borate and articles comprising such metal borates.
Beta-barium borate (.beta.-BBO) is one of the materials having non-linear optical properties, and more particularly having the capacity in a certain range of wavelength, to produce a second harmonic generation of frequency 2w when it is subjected to incident radiation of frequency w. Unlike most other materials with usual sources of incident radiation, this second harmonic generation (SHG) is of sufficient amplitude to be observed and manipulated.
Another advantage of .beta.-BBO is that its optical damage threshold is high, i.e. 3-4 times the threshold of potassium dihydrogenphosphate, which is used as a reference material in non-linear optics. This means that .beta.-BBO can be used with short pulse, high power lasers as sources of incident radiation without being damaged. Also, .beta.-BBO can be used for non-linear optics within a range of temperatures which is broader than for any other material.
To be actually usable in non-linear integrated optical devices, however, a material needs to be obtainable in single crystal or in crystalline thin film forms, on appropriate substrates.
Various methods are known to prepare such thin films on substrates. Most of these methods are complex, expensive, provide non-uniform films with uncontrolled stoichiometry and do not permit the easy introduction of dopants in a quantitatively controlled manner. Some of these problems may be partially overcome by tedious and time consuming trial and error combinations of the chemical elements in question in order to achieve thin films with the desired composition ratios. Finally, it is generally difficult to deposit thin film on irregular geometries.
Another well known approach for the deposition of thin films is by metallo-organic decomposition (MOD). MOD for preparing inorganic films is described in U.S. Pat. Nos. 3,330,697 and 4,480,770. MOD consists in depositing a solution containing metal precursors in the desired ratios, onto a substrate. These precursors are then thermally decomposed to form an inorganic thin film. Further thermal treatments result in the formation of the desired phase. However, a requirement for the MOD method is to find precursors which decompose quantitatively and are not water-sensitive. MOD was thought not to be a good candidate for preparing thin films of .beta.-BBO, since materials generally known as boron precursors are water-sensitive and do not decompose quantitatively.
The present invention eliminates the above difficulties and provides a way to use the MOD method for preparing metal borates and especially thin films of metal borate such as .beta.-BBO with precise atomic ratios on various substrates and without requiring expensive and complex deposition systems or the preliminary preparation of powders of the desired material.